The present invention relates to semiconductor manufacturing technology, and, more specifically, to measuring strain of epitaxial films in relatively small pads using micro-spot x-ray diffraction for in-line metrology during the semiconductor manufacturing process.
X-ray diffraction (XRD) is commonly used as a viable measuring technique in the research and development of epitaxial deposition processes in semiconductor manufacturing. Until recently, relatively large area sample sizes (e.g., up to several centimeters squared) were required to successfully carry out an XRD measurement due to x-ray signal strength and spot size. However, the development of new systems and, in particular, intense micro-spot XRD sources have enabled the capability of probing modern standard size measurement pads as typically employed in the industry. Typical modern pad sizes range from 50×50 um2 to 500×500 um2 and state-of-the-art x-ray spot dimensions are approximately 70 um (oval shaped—due to sample tilt). This has enabled x-rays to be used for in-line metrology during the semiconductor manufacturing process. Common parameters that are measured using XRD include the strain, concentration and thickness of a doped silicon pad (e.g., Si(1-x)Ge(x), Si(1-x)C(x) where x is the amount of substitutional dopant) that is part of a patterned wafer having a plurality of such pads. Typically the dopant is used to stress the channel of a semiconductor device formed in the wafer. The patterned wafer could be bulk silicon or a silicon-on-insulator (SOI) wafer (needed for high performance devices). Even though the dopant layer may be SiGe, any kind of dopant, e.g., B, C, etc., may be utilized.
However, problems with the modern micro-spot XRD sources include the fact that source intensity is reduced and relatively much longer counting times are usually needed. The measurement of the strain starts with alignment of the wafer to the XRD beam. Bulk silicon wafer alignment of the XRD source to the peak diffraction area of the silicon is straightforward because of the relatively large diffraction signal intensity from the substrate. Alignment for measurement of peak diffraction on SOI wafers, which is important for modern high-performance technologies, is more complicated, depending upon the process used for manufacturing the wafers. SIMOX wafers have top silicon layers that exhibit the same orientation as the substrate, and thus, a substrate alignment of the XRD source is sufficient as a scan set up. On the other hand, bonded SOI wafers inherently can have miscut or relatively slight angular tilt between the handle and the upper silicon wafer before bonding. As a result, the substrate crystal orientation can differ from that of the upper silicon layer by twice the manufacturing tolerance. X-ray diffraction is extremely sensitive to crystal orientation, and a mere alignment of the XRD source to the base silicon substrate peak diffraction area is insufficient. A relatively well-defined routine may be executed to find the location of the SOI peak position of the upper silicon layer (which is located above the buried oxide layer, which itself is located above the relatively thicker bulk silicon substrate layer), after which the system moves to that angular location and begins the scan. However, for SOI devices with upper silicon layers having a thickness starting as low as 150 Angstroms, it becomes extremely difficult to achieve alignment of the x-ray beam for strain measurements since the diffracted intensity is several orders of magnitude lower than that of the bulk silicon.